Metal casting process



United States Patent 15 Claims. (Cl. 22216.5)

The instant invention relates to an improved metal casting process. More specifically, the present invention is concerned with a method of increasing the life of mold bottoms used to cast ingots, and prevent adherence of these mold bases to subsequently formed solidified ingots which must be removed from within the mold walls.

All metal ingots are cast from molds. One popular type is a big end down mold. These molds rest on bases commonly known as stools. The stools are merely large, normally rectangular, flat slabs of metal, commonly made of cast iron, which are used as support for the mold sides and also, of course, form the bottom portion of the mold. The mold sides generally taper up in diameter from bottom to top. Another type of mold is known as the big end up mold. These are ladlelike receivers for the molten metal, the bottom portion of which molds are integral, nonremovable parts of the entire mold.

Various problems commonly occur in use of these molds and particularly with respect to the surface of their base portions. First, the unprotected metal surface quickly erodes and pits in the presence of molten metals which are cascaded upon their surface. Large gouges in the base portions are produced due to the force and high temperature developed [by the flowing molten metal which contacts the surface of the stool. Since many molds are generally approximately 5-10 feet in height, the metal must be poured from a height at least equal to that distance and quite often is poured from even greater heights. A considerable pressure head is thereby developed. Thus, the hot molten metal easily gouges gaping depressions in the base members under such force and at a temperature of at least the liquefaction temperature of the molten metal. Moreover, the problem of creation of pits or gouges in the base portions of the molds, caused by the above factors is aggravated due to the fact that the molten metal, especially near the bottom of the mold, remains in its erosive hot liquid state for a considerable amount of time subsequent to pouring.

The molten metal upon solidification to an ingot thereby has a bottom form conforming to the undesirable eroded surface configuration of the stool or base member of the mold. Thus, a considerable amount of the ingot, when withdrawn from the mold and subsequently processed into slabs or blooms, is lost through a cropping of the irregularly formed end of the slab. This, of course, is highly undesirable, since it results in undue loss of usable metal and increase in scrap, which must be subsequently reprocessed.

Another extremely serious and costly problem results after the ingot in the mold has solidified to a point where it can be removed from both the mold sides and its base platform member or stool. In ,many cases, if the surface of the stool is unprotected, or inadequately protected, and erosion occurs as described above, the ingot has a greater tendency to remain tightly adherent to the stool. Thus, after the mold sides are removed from around the ingot, which process can normally be efficiently achieved with a minimal film of coating selected from a variety of coating agents, the ingot must be forcibly removed from the stool. This is normally achieved by raising both ingot and adherent stool, and

3,184,315 Patented May 25, 1965 lot:

thrusting them against some other larger object Where by the ingot is jarred loose. In many cases the stool and ingot are merely dropped on the floor from some suitable height. In such a situation, the stool is often broken into two or more smaller pieces and cannot be subsequently reused in casting other ingots. Again, replacement cost of these stools is high, making this aspect of the overall casting process somewhat disadvantageous. The same problem exists with respect to big end up molds wherein sticking of ingots particularly occurs at their base portion. New molds of this type are especially vulnerable to sticking due to their smooth surface unprotected by any layers of metal oxides or scale. A tight metal-to-metal bond between mold bottoms and ingots then occurs.

Cracking of molds and particularly their base portions due to the above discussed rough handling occasioned by stickers between the base portions and ingots, is also enhanced by thermal shock during ingot formation. Unprotected or inadequately protected bottom surfaces of mold are especially susceptible to such destructive shock.

Many prior art coating materials have failed to give adequate protection to the mold [base members, and in some cases caused ancillary process difiiculties. For example, some inferior coatings were washed off the surface of the base portion of the mold and were thereby included as an unwanted impurity in the ingots. Such inclusion oftentimes deleteriously affected desirable metal properties.

The above described problem is a particularly arduous one, which prior art coating materials have not been able to cope with to any substantial degree. It would therefore be of benefit to the art it a method were discovered whereby the surfaces of the above described stools of big end down molds or base members of big end up molds, could be protected from erosion from cascading molten metal. If such erosion could be substantially prevented and, in addition, the problem of adherence between the base member and subsequently formed ingot could also be overcome, considerable advantages would accrue. For example, the overall casting process would be materially benefitted since the separation between the mold base member and ingot would be accomplished in a more facile operation.

It therefore becomes an object of the invention to provide a method of casting metal ingots into metal molds whereby adherence of the base member of the mold to the formed ingot and erosion of the same base member during ingot formation are substantially prevented.

A specific object of the invention is to inhibit such erosion and adherence by applying a film of coating material to the surface of the base member whereby these problems are overcome.

Yet another object is to provide improved metal molds used for casting metal ingots which have at least the base member of the mold protected by a film of material so that the above described problems of erosion of the base member and adherence of same to metal ingots no longer exist to any appreciable degree.

Other objects will appear hereinafter.

In accordance with the invention a method of casting metal objects from metal molds has been discovered, whereby adherence of the base member of those molds to ingots formed therefrom, and erosion of these same base members during such ingot formation are substantially inhibited.

In its broadest aspects, the invention comprises the steps of applying a slurry consisting of an aqueous alkali metal silicate binder and a refractory to the surface of the base member of a metal mold. Preferred binders include sodium silicate and potassium silicate. Among these, the most preferred by virtue of its availability and low cost is sodium silicate.

In order to best achieve dual purposes of prevention of erosion of base member and non-adherence to formed ingots, the slurry should consist of at least one refractory of the vfollowing types: vitreous silica, crystalline silica, magnesium silicate, aluminum silicate, alumina, graphite, zirconium silicate and clay. These materials are all well known substances and are all commercially available. Typical aluminum silicates, for example, may include mica, a laminated type of aluminum silicate, and mullite, an orthorhombic aluminum silicate available from the Island of Mull or artificially made by heating andalusite, sillimanite or cyanite. Excellent magnesium silicates are forsterite or talc, While a useful zirconium silicate is zircon. The most preferred refractories, discussed in more detail hereinafter, are vitreous and crystalline silicas.

The slurry is provided in an amount adequate to form a coating of sufficient thickness to prevent the above mentioned adherence and erosion from occurring. After the slurry is allowed to dry, with or without application of heat, whereby the liquid phase is driven from the surface of the base member, leaving a thin film of solid refractory' coating, the molten metal is thereafter poured into the mold and on top of the now coated base member. The liquid metal is allowed to solidify into an ingot and then removed from the coated base member and mold sides. ,The slurry coating reagent is simply applied to the stool or bottom of the mold by a wide variety of methods. For example, the'slurry may be applied by flowing it over the stool, by spray techniques, by coating the stool with some type of applicator, etc. Spray application is believed to be the most efiicient' and practical way of slurry application. The slurry is best applied to the stool .por tion of big end down moldsbefore the metal mold sides are placed thereon. I V

Etiecting removal of the liquid phase of the slurry from the solid refractory material may likewise be carried out in a variety of methods. For example, the bottom portion of the mold may be coated with the slurry and allowed to dry gradually. Another method of laying 4 ing is complete except when. big end down molds are used. In those cases the mold sides should be placed on the base member following coating,.if not already present during coating.

After coating operations, molten metal is flowed into the mold,-allowed to solidify into an ingot and the ingot is then separated from mold surfaces. The invention is not limited to use with specific mold sides or any particular metal mold bottom or to use with any particular molten metal. However, it has found specially preferred use in coating cast iron metal base members for molds which are used in forming ingots of steel.

As mentioned above, two refractories have shown extreme promise for coating. Both of these materials are well-known and need little further elaboration. The first is a crystalline silica refractory. Representative types include sand, crystalline silica flour, crystalline silica grains, etc.

Other preferred refractory materials are those generally referred to as vitreous silicas. These are glassy modifications of silica, obtained by the v fusion of selected lowtemperature crystalline forms, and are frequently referred to as quartz glass or silica glass; Specific vitreous silicas include those particle's made from fused quartz glasses, silicate glasses, silica glasses such as the .well-known Vycor materials and fused silica glasses. With respect to all of these materialstheir thermal coeflicients of expansion are relatively small in proportion to expansion down a thin protective coating is to apply the slurry to e 7 period.

'Another way of applying the protective film is'to coat the base member, and then heat it as slowly or rapidly as a desired to drive off the liquid portion of the slurry. In

any event all that is necessary is that the coating be laid down in some manner after contact of the base member with the slurry.

In the most preferred method the coating slurry is applied to stool or mold base members having a temperature ranging from that of room temperature to 1000 F., and more preferably from 200.F. to 800 F. Best adherence of solid coating to stools is achieved by slurry application to the stools at a temperature range of 200- 500 F. For best results, it has been determined that films should measure in thickness from 0.01" to 3" and most preferably from 0.01" to Vs".

It is believed that the excellent coating success achieved by. use of the above described slurries is their ability to form a strong ceramic coating even when afiixed to the base members of the mold at relatively low temperatures, at least under foundry conditions, of say about 500 F; or even lower. The coating becomes completely resistant to subsequent contact with Water and stays tightly adherent to the base portions of the mold even under such washings.

After the coating has been formed preparation for east- This is particularly preferred in that the stools properties of other refractories such as those of the soda:

lime and lead glass types; Generally, :thejabove pres.

ferred refractories have thermal expansion coeflicients smaller than 5X10 cm./cm./ C. Also, the silica content of these granular siliceous refractory materials is generally greater than 9.6% silica expressed as SiO and may range as high as 99.8% SiO Thus, by the term vitreous silica is meant, a refractory comprising a silica glass having a thermal coefiicientof expansion and SiO content within :the above ranges.

It has been determinedfthat for best results in coating stools the refractory used in the silica slurry should be able to Withstand severe heat'shocks. Due to the extreme hot temperature of the molten metal as compared'to that of the stooleven when thelatter is heated, an exceedingly abrupt change in temperature occurs when the metal contactsthe stool. The coating must itself be able to withstand this heat shock to impart necessary protection to the stool base. It has been theorized that failure of some prior art materials was due, at least in part, to their inability to withstand this sudden increase in heat, thereby resulting in cracking of the coating and subsequent exposure of the metal surface to the cascading molten metal pouredinto the mold;

In view ofthe. above it is generally thought that the most preferred silica-type refractories are those 'which' have the highest silica content concomitant with the lowest thermal coefiicient of expansion. These properties are particularly possessed by vitreous silicas'and more'particularly those of the fused silica types. The latter mate rials have a silica content greater :than 97% I silica expressed as Si0 and a thermal coefficient of expansion not greater than about 6x10 'cm./cm./ C. i

A typicalfused silica of the type described above which is extremely useful in the practice of the invention, having a thermal coefficient of expansion of about 5x10 cm./cm./ C., has the following typical analysis:

TABLE I Ingredients: I Percent by weight 510 97.3 A1203 -Q- Sub-oxides of silica Q 1.0

The above type silica products are readily'prepar'ed by grinding very pure fused silica glasses. Likewise, the bore-silicate glasses and Vycor silica glasses may also be ground to produce extremely useful refractories. Vitreous silica substances marketed under the trademark, Nalcast have been employed with much success in preventing erosion of stools and adherence of same to the formed ingots.

' As set out above, the preferred source of silicate material is a sodium silicate. Commercial sources of this material range from about 25% to about 40% by weight of solids of sodium silicate, generally supplied in aqueous form. More typically, they contain 30-40% by weight of solids. These commercially available silicates may be concentrated somewhat or diluted to any solids content as desired. Preferred alkali metal silicate binders for use in the invention are those materials containing 5-45 by weight of alkali metal silicate as a water solution.

' While water alone may be used as the solubilizing agent for'alkali metal silicate to form the liquid binders of the invention, it is understood that a binder may contain other solvents, polar in nature. It is only necessary that the organic solvents be compatible in water and do not deleteriously affect the alkali metal silicate, such by initiating polymerization of it to a solid, unusable, glassy-mass. Particularly preferred organic substances used in combination with Water as solvents for the alkali metal silicate material are those which lower the freezing point of pure aqueous alkali metal silicates by their mixture with aqueous silicate solutions. Such products are especially useful during the colder months of the year when they must be stored and/ or used at relatively low temperature. Amines such as morpholine, di-ethyl amine, etc.,and polyhydroxy organics as ethylene glycol, glycerine, etc., are preferred materials in making up solutions of alkali metal silicate binder. A preferred binder, winterized against freezing con- ,tains 5-45 by weight of alkali metal silicate, -95% of water, and 5-50 by weight of a polyhydroxy water compatible polar organic compound.

Regardless of whether the binder is a relatively pure aqueous solution of alkali metal silicate or a combination solvent system involving both water and water compatible organic solvents of the type described above or others, the ratio of refractory to binder in making up the slurry products ofthe inventionranges from 2:1 to 1:50. The more preferred slurries contain refractory and more preferably-crystalline silica or vitreous silica suspended in one -or more -of the above described binders-above in a ratio of from 2:1 to 1:25.

The particle size of the refractory may vary over a wide range, as long as it is able to'be dispersed in the alkali metal silicate binder. It is preferred, however, that the refractory particles be sufficiently small so that a uniform,

relatively long lasting dispersion of refractory in binder may be made. The smaller the particle size, the longer aslurry made up of binder and refractory remains in a homogeneous state. It has been determined that refractory particles ranging in size from 150 mesh to as low as a fraction of a micron may be employed. The most preferred refractory materials have average particle diameter size below about 350 mesh, with particles corresponding stituting a portion of the binder contained 37.5% solids,

with the remainder being water. It is understood, of course, that these examples are merely illustrative and that the invention is not limited to use of these alone. The percentages expressed are percents by weight.

Example I Percent Sodium Silicate 50 Crystalline silica flour 50 6 Example 11 Percent Sodium Silicate 25 Water 25 Crystalline silica flour 50 Example Ill Percent Sodium silicate 10 Water 40 Crystalline silica flour 50 Example IV Percent Sodium silicate 30 Water 30 Crystalline silica flour 40 Example V 7 Percent Sodium silicate 20 Water 20 Crystalline silica flour 60 Example VI Percent Sodium silicate 20 Water 75 Crystalline silica flour 5 A coating slurry was made up having the following proportions:

Material: Pounds 37.5 aqueous solution of sodium silicate--- .117 Water 28 Ethylene glycol 3 l Crystalline silica flour Bentonite 3.5

The bentonite, used as. a dispersing agent, was mixed with about 3 volumes of the crystalline silica powder and added slowly to the sodium silicate which had been further diluted with the water. Stirring was effected for 15 minutes, after which time the ethylene glycol was added. The remainder of the crystalline silica powder was then rapidly stirred until a smooth suspension was obtained.

The above slurry was then sprayed onto a total of -17 castiron stools which were approximately 7' x 4' x 18 in dimension. The weightof each of these stools, which, after coating acted as mold bottoms, was approximately 20,000 pounds. Approximately;% gallon of slurry was supplied per stool. Since the stools, when coated, were heatedto a temperature of 200-400 R, the drying of the slurry by driving off the liquidphase, and deposition of siliceous film occurred in a matter of a few seconds. After the film had been applied, open bottom-open top metal mold sides were placed thereon, having approximate dimensions of 92" in height and whose sides measured 38" and 48" respectively. The entire mold was then sprayed with water in order to cool it sufiiciently to allow personnel to also spray the Walls of the mold with a carbonaceous coating. At the same time the mold walls were cleaned of any adherent slag, metal, etc. After these operations were complete, a low carbon steel made by the Basic Oxygen Process was poured into the mold containers from a 3" diameter hole in the ,bottom of a ladle.

In all cases the coating remained intact, even under the erosive process conditions of pouring of molten metal from a substantial height through anopening of relatively smalldiameter, thereby developing substantial head a age-15 was formed over the coated stool before a metal skin" formed; Thus, any gouges and pitting produced: by the erosive falling effect of the molten metal would be greatly aggravated by the molten metal, remaining in contact with the stool for a considerable length of time before solidification.

However, in the instant experiments, in no case did there appear any appreciable sign of erosion. Also, some of the stools could be reused in a number of runs before additional coating had to be applied. Moreover, after the metal had solidified and the ingot was removed from the mold sides and'stool, no stickers appeared. That is, there was substantially no adherence of the stoolor mold base tothe newly solidified ingot, which could be easily removed with a minimum of process time. Also,

since no erosion did occur, the amount of butt of the ingot was-minimized, and the amount of scrap lost upon subsequent slabbing was substantial-1y reduced. 1 In other runs inwhichthe fall of molten metal on to the stool by form a tightly adherent, continuous and unbroken prisedwof' 5-45 by weight of alkali'metal silicate, '10.- r

coating whichis securely bonded to the: surface of the,

stool. WithOut proper amount of binder in relation to.

refractory, the resultant coating, after application and drying of slurry, exhibits a pan-cake eifect with numer-- ous undesirable holes appearing in the coating, thereby exposing portions of the stools surface. In those cases in which proper ratios of binder to refractory were not followed; the whole coating loosened itself from the stool surface even before molten metal pouring. began;

:It canbe seenfrom the above experiment that the.

overall objectives of substantially preventing erosion of stools or base-members'of -rnetal molds during pouring of molten metal thereon, and prevention of adherence ofthef subsequently formed solidified ingot to the base member have been achieved to a substantialdegree. In

addition, subsidiary .good effects are to benoted. For

example, since .erosion has been reduced to a minimal 7 effect, stool" life issubstantially increased. I Likewise,

since no. adherence of the stools to the ingots occurs, no

ting down on track Weightand, truck maintenance, since stools are normally carried by means ,of flat cars on rails "torthe point of pouring of metal.vv Also,-since the coating appears to withstand a wider-ange ofjternperature of molten metal, it is possible that severe temperature su'r-t face applications may be performed which were heretofore impossible due to failure .of prior art coatings- Many other advantages ofthe mold forming'process of the in- Mention are apparent,

The invention is hereby claimed .as follows:

l.'A method of inhibiting erosion of the surface of the base member of a metal moldused to cast ingots,

which erosion normallyoccursduring contact of said sufficient thickness. to inhibit; said erosion and-prevent saidadherence, and allowing said slurry to dry'whereby a protective solid coating is formed uponsaid base memher, said binder being present in an amount suflic'ient to bind the refractory particles together to thereby form a tightly adherent coating bonded to said, surface.

2. The method of claim 1 wherein said base member is cast iron, and said ingots cast. are composed of steel. 3. The method of claim 1 wherein saidaqueous binder comprises 5-45% by'weigihtof alkali metal silicate, and the ratio of. saidrefractory to saidaqueous alkali metal silicate binder ranges from 2:1 to.l:50.

4. The methodrof claim 3 wherein said binder is com} 95% by. weight of water and 5-.5 0% by weight of a polyhydroxy water compatible organic compound;

5. The method of claim 1 wherein :said refractory is crystalline silica. I

6. -The method of claim 1 wherein said refractory is vitreous silica. r i

7. In the method of casting metal ingots from a metal mold wherebyadherence of the-base member of-said resort need be'taken to physically contacting the two adherent articles against a third larger object such as a floor, to separate the united objects and recover the ingots- As mentioned above, not only is this separation procedure time. consumingand costly, but it frequently results in cracking or complete disintegration of the, stool,

from the ingot is substantially reduced. As an overall advantagdthe stool inventory necessary for eflicient opert at-ion may be substantially. reduced throughuse of the.

by formation ofthe coating film, more ductile stools may be utilized. Thus, lighter stools may be employed, cut-.

. making itcompletely unsuitable for further employment. Also, since little or no'erosion occurs due to. cascading molten metal, theamonnt of butt required to be cropped mold to said formed ingots and erosion of said base memher during formation of said' ingots are, substantially prevented; Whichcomprises the steps of applying 1501116 surface of said base member leaving athi'n film of refractory selected from'the group consisting of vitreous silica, t

crystalline silica, magnesium silicate, aluminum silicate; alumina, graphite, zirconium silicate and clay suspended inan aqueous alkali metal silica-tebinder, such slurry being applied in an amout adequate to'forma solid coating of suflicient thickness to inhibit said erosion and preventsaid adherence, allowing such slurry todry whereby the liquid. phase. of such slurry is driven from theJsur-.- face of said :base member leaving 'a thin film ofrefractoryppouring molten metal into said mold, allowing said metal to solidify-into an ingot, 'and removingw said ingot from'said coated base member in said mold; said binder being Ipresentin :an amount'sutlicient to .bindthe refracto-ryv particles together to thereby form a tightly adherent coating bonded to said surface. i

8.'The'method of claim17 wherein said coated base member iscomposed of cast-iron andsaid ingotsare steel.

9. The method of. claim 7 wherein said.. aqueousbinder comprises 545% by weight of alkali, metal, silicate, and

. the ratio of said refractory'to said aqueous binder ranges from 2: 1 to 1:50. I I. m

10. The method ofclaim 9 wherein said aqueous binder comprises 545% by weightof-alkali metal silicate, 10- of water, and 5-"50% by weight of a polyhydroxy water compatible organic compound.

11. Themethod of claim Twherein said refractoryis vitreous silica.

12. The method of claim 7 wherein said refractory is crystalline silica,

13. An improved metal mold for casting metal ingots which comprises an open-top mold having at least its base member coated with a thin solid protective film of a refractory material derived from drying a slurry comprising a refractory selected from the group consisting of vitreous silica, crystalline silica, magnesium silicate, aluminum silicate, alumina, graphite, zirconium silicate and clay suspended in an aqueous alkali metal silicate binder, said binder being present in said slurry in an amount efficient to bind the refractory together to thereby form a tightly adherent coating bonded to said base member, and said coated *base member being further characterized as being non-erosive to flowing molten metal and nonadherent to subsequently formed solid metal ingots.

10 14. The metal mold of claim 13 wherein said coating is derived froma slurry comprising an aqueous binder containing 5-45 by weight of alkali metal silicate and a crystalline silica refractory in a ratio to said binder of from 21-1 to 1:50.

15. The metal mold of claim 13 wherein said coating is silica.

References Cited by the Examiner UNITED STATES PATENTS 2,623,809 12/52 Myers 22-192 2,948,032 8/60 Renter 22193 3,059,296 10/62 North 22-193 3,077,413 2/63 Campbell 22-*193 3,116,524 1/64- Royal 22-192 MARCUS U. LYONS, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,184,815 May 25, 1965 Raymond Reuter It is hereby ce tified that error appears in the above numbered patent reqliring correction and that the said Letters Patent should read as correctedbelow.

Column 8, line 47, for "leaving a thin film of" read a slu ry comprising a Signed and sealed this 26th day of October 1965,

(SEAL) Atlest:

ERNEST W. SWIDER EDWARD J. BRENNER Atlcsting Officer Commissioner of Patents 

1. A METHOD OF INHIBITING EROSION OF THE SURFACE OF THE BASE MEMBER OF A METAL MOLD USED TO CAST INGOTS, WHICH EROSION NORMALLY OCCURS DURING CONTACT OF SAID SURFACE WITH FLOWING MOLTEN METAL, WHILE SUBSEQUENTLY PREVENTING ADHERENCE OF SAID INGOTS TO SAID BASE MEMBER UPON INGOT FORMATION; WHICH COMPRISES THE STEPS OF APPLYING TO SAID SURFACE A SLURRY COMPRISING A REFRACTORY SELECTED FROM THE GROUP CONSISTING OF VITREOUS SILICA, CRYSTALLINE SILICA, MAGNESIUM SILICATE, ALUMINUM SILICATE, ALUMINA, GRAPHITE, ZIRCONIUM SILICATE AND CLAY SUSPENDED IN AN AQUEOUS ALKALI METAL SILICATE BINDER, SAID SLURRY BEING EMPLOYED IN AN AMOUNT ADEQUATE TO FORM A COATING OF SUFFICIENT THICKNESS TO INHIBIT SAID EROSION AND PREVENT SAID ADHERENCE, AND ALLOWING SAID SLURRY TO DRY WHEREBY A PROTECTIVE SOLID COATING IS FORMED UPON SAID BASE MEMBER, SAID BINDER BEING PRESENT IN AN AMOUNT SUFFICIENT TO BIND THE REFRACTORY PARTICLES TOGETHER TO THEREBY FORM A TIGHTLY ADHERENT COATING BONDED TO SAID SURFACE. 